Note: Descriptions are shown in the official language in which they were submitted.
THE PRESERVATION OF WOOD
Field of the Invention
This invention relates to the preservation of wood,
and includes the chemical compositions used to treat the
wood, the process of treating the wood, and the treated wood
that results from the process. The chemical compositions
which are described herein are especially well suited for
use in the method of U.S. 3,968,276 in which wood to be -treated
is exposed to a reduced pressure for the purpose of removing
moisture and volatiles from the wood, and thereafter impregnating
the wood with an aqueous chemical composition.
Background of the Invention
Wood is one of the oldest materials known to man. Its
ready availability in most parts of the world, coupled with
the ease with which it can be cut and fabricated into useful
articles makes it well suited for a variety of applications.
However, because wood is a cellulosic material it is susceptible
to decay and deterioration over any extended period of time.
As a result, numerous methods have been proposed for treating
wood to improve its durability, particularly when used under
conditions where the wood is exposed to moisture or biological
attack.
Applications of wood such as its use in manufacturing
railroad ties, utility poles, fence posts, and the like have
created substantial incentives for industry to develop effective
and commercially practical methods of treating the wood to
extend its useful life (i.e. wood preservation). Various
techniques of wood preserva-tion have been used including
surface coating, surface treating, and pressure penetration
of the wood with a variety of chemicals such as creosote,
pentachlorophenol, and the like.
For example, halogenated phenols such as pentachlorophenol
are often used to treat wooden poles (such as utility poles).
When used for this purpose, it is common to dissolve the
pentachlorophenol in a hydrocarbon oil, pressure treat the
poles, and then evaporate the excess oil. Such a process
is relatively expensive, but is justified by the wood preserva-
tive properties of the pentachlorophenol. However, the treated
poles tend to "bleed" their treating solution into the surround-
ing soil when the poles are installed in the ground. This
draining or leaching effect is more visible on the sunny
side of utility poles which drain or bleed faster because
the heat from the sun warms the treating solution and reduces
its viscosity. In an effort to reduce treating costs, water
is sometimes substi-tuted for the hydrocarbon oil, and the
pentachlorophenol is used in one of its salt forms to provide
water solubility. However, this approach has a number of
disadvantages and is less preferred than the oil treatment.
Unfortunately, most of the present trea-ting processes
(e.g. those using creosote, various copper and mercury compounds,
or pentachlorophenol), even when operated under significant
pressure conditions, do not result in deep impregnation of
the wood. Thus, the wood being treated with any chemical
is typically treated to only a shallow depth and, in the
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case of logs or poles very of-ten the degree of pene-tration
does not extend throughout all of the soft or sapwood, much
less penetrate the heartwood. Except for one known process,
none of the known commercial processes for preserving wood
by chemical -treatment involves penetrating the wood poles
or logs more than perhaps a few millimeters into the heartwood.
The one known exception is the process of U.S. Patent 3,968,276.
According to U.S. Patent No. 3,968,276 wood (logs or
dimension lumber) is preserved by impregnating the wood with
chemicals which penetrate substantially to the core or center
of the wood being treated. In the case of logs or poles,
this penetration can be deep within the heartwood and may
be substantially complete. This is accomplished by enclosing
the wood within a chamber, sealing the chamber, evacuating
the chamber slowly so that the wood is exposed to a pressure
below atmosphere pressure, introducing into the chamber a
sufficient amount of an aqueous solution of resin prepolymer
to immerse the wood and then maintaining the vacuum over
the immersed wood for a period of time both to allow the
removal of moisture and volatiles from the wood and to permit
the treating solution to enter into the wood. Thereafter,
an additional amount of the resin prepolymer solution is
introduced into the chamber (ie needed) and the pressure
on the system is increased to superatmospheric pressure to
thereby cause an even deeper penetration of the treating
solution into the wood. The vacuum and pressure cycle may
be repeated one or rnore times (e.g. 1 to 30 or more cycles).
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When the treatmen-t is completed to the desired exten-t, the
treated wood is recovered from the chamber.
The process just described is extremely effective and
achieves substantially greater penetration of the wood than
other commonly practiced -treating processes.
It would be beneficial to industry if common wood preserva-
tives such as pen-tachlorophenol could be introduced deep
within the wood being treated and if the bleeding or leaching
of the pentachlorophenol could be reduced.
Summary of the Invention
-
The present invention is based on the discovery that
one or more known wood preservatives, such as the halogenated
phenols, can be introduced deep into the wood being treated
and obtain reduced "bleeding" or leaching, by combining the
lS wood preservative with the method of U.S. Patent 3,968,276
by using a special class of treating compositions that are
water based or water dilutable which permits the benefits
of the aqueous resin prepolymer system of U.S. Patent No.
3,968,276 to be maintained. Fur-ther, it has been discovered
that it is possible to accomplish this without encountering
any significant amount of "blooming" which is normally associated
with the use of aqueous treating solutions of halogenated
phenols, such as pentachlorophenol in the treatment oE wood.
This blooming effect is caused by the fact that the aqueous
treating solutions are made from salts of halogenated phenols
(such as the salts of pen-tachlorophenol) because the salts
dissolve readily in wa-ter. As a result, after the phenate
~ .
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salts have been used to treat wood, there is a tendency for
the salts to be redissolved by rain water or ground water
and -to migrate easily to the surface of the treated wood
where they crystallize during the subsequent evaporation
of the water.
Briefly described, the present invention involves manu-
facturing a water dilutable concentrate or a water based
treating solution which consists essentially of:
(a) a halogenated phenol wood preservative such
as pentachlorophenol;
(b) a phenolic resin prepolymer such as a phenol
formaldehyde prepolymer;
(c) a coupling or solubilizing agent for enhancing
the water compatibility of the concentrate (e.g. methyl
alcohol or acetone with or without the addition of a
surfactant); and
(d) water.
Optionally and preferably, the treating solutions will
also contain organo/metallic salts, especially copper or
zinc salts.
In addition, it has been discovered that the ability
of the prepolymer to cure rapidly and lock in the halogenated
phenol deep within the wood can be enhanced by heating the
wood at the conclusion of the treatment (e.g. heating the
wood with steam) to thereby accelerate the curing of the
resin prepolymer.
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Detailed Descrip-tion
The presen-t invention concerns water based so]utions
for treating wood and the use of such solutions to treat
wood (both dried wood and green wood), and the resulting
treated wood products. For convenience, it is sometimes
convenient to make the treating solutions in two steps, the
first step being the preparation of water-dilutable concentrate
or syrup, and the second step consisting of diluting the
concentrate with water to achieve a desired concentration
of active ingredlents. Although any distinction between
the concentrate and the aqueous treating solutions, per se,
is to a certain extent arbitrary, the treating solutions
will normally contain from 1-40% (e.g. 2-30%) by weight of
the combined amount of the halogenated phenol wood preservative
and a phenolic resin prepolymer, more usually about 5-25%
on the same basis. By comparison, the water dilutable concen-
trates will usually contain from 10-65% by weight of the
combined amount of halogenated phenol wood preservative and
phenolic resin prepolymer, usually within the range of 15-
50% on the same basis. Of course, the use solutions maybe prepared directly.
The ingredients which are used in preparing the aqueous
treating solutions of the present invention and the corres-
ponding water dilutable concentrates are as follows.
The first essential ingredient is a halogenated phenol wood
preservative. Typically, the halogenated phenol wood preserva-
tive will be a chlorinated phenol~ Pentachlorophenol is
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preferred. Commercial grade pentachlorophenol (which con-tains
related compounds such as tetrachlorophenol) is particularly
useful. These halogenated phenol wood preservatives will
ordinarily be used in their phenol form and not as a salt
as is conventionally done in the case of trea-ting wood with
pentachlorophenol where the phenol is used as the phenate
salt because it dissolves readily in inexpensive polar solvents
such as water.
The resin prepolymer used in the present compositions
will be a water compatible phenolic resin prepolymer, preferably
a water based phenol formaldehyde prepolymer. Water soluble
or dispersible phenolic resin prepolymers are well known
and their preparation, per se, forms no part of this invention.
However, all phenolic resin prepolymers do not work with
equal effectiveness, and the aqueous phenol formaldehyde
prepolymers are pre:Eerred. Low viscosity prepolymers are
preferred for ease of use. Such prepolymers typically have
molecular weights (weight average) of less than 1000 (e.g.
80 to 500).
Although the ratio of halogenated phenol wood preservative
to phenolic resin prepolymer will vary depending upon a variety
of factors including the particular ingredients being used,
the purpose of the wood treatment, the type of wood being
treated, etc., in general the wei~ht ratio of halogenated
phenol to resin prepolymer will be within the range of from
3:1 to 1:15, and more preferably within the range of 1:2
to 1:10.
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The treating compositions of the present invention (as
well as the concentrates) will also contain a coupling agent
or solubilizing agent for enhancing the water compatibility
of the mixture of halogenated phenol and resin prepolymer.
This coupling agent will typically comprise a polar co-solvent
such as alcohols and ketones. Alcohols such as Cl-C~ alkanols
(e.g. methyl aIcohol) are preferred, with or without added
surfactants (e.g. sufactants of the polyethoxy type). The
amount of surfactant will usually be 0 to 5 weight-% (e.g.
0.1 to 3 weight-%), based on the total composition weight.
The amount of coupling agent used will depend on the coupling
agent that is used and the amount of halogenated phenol present.
The trea-ting compositions of the present invention (as
well as the concentrates) will also contain water as an essential
ingredient. In the dilute or use solutions, the water content
will generally be at least 50% by weight of the treating
solution, usually 55-95% by weight (e.g. 60-75% by weight).
In the case of the concentrates, water will normally be present
in amounts of 50% by weight or more of the weight of the
coupling agent (e.g. methanol with or wi-thout added surfactants).
If desired, the treating compositions of the present
invention and the concentrates may contain other ingredients
such as fire retardant chemicals, dyes or colorants, ca-talys-ts,
various biocides, and other chemicals (e.g. borax whlch serves
as a buffer or stabilizer). The presence of other wood preserva-
tives such as the organo-metallic compounds is optional but
preferred. Compounds of zinc and copper are particularly
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desirable. Suitable organo/metallic compounds include ~inc
naphthenate, copper naphthenate and copper-8-quinolinolate.
To prepare the treating compositions of the present
invention (and the concentrates), the preferred procedure
is to dissolve the halogenated phenol (e.g. pentachlorophenol)
in the alcohol, or other coupling agent, and then slowly
adding to the resulting mixture an aqueous resin prepolymer,
(usually with vigorous agitation), then adding the surfactant
(if one is used), then the metallic compounds (if any), and
then finally adding such additional water as may be appropriate,
noting that the total amount of water to be added will vary
depending upon the desired strength oE the final product
and depending upon whether or not one is interested in producing
only a concen-trate for convenient shipment to a distant point
of usage or whether one wishes to prepare the final composition
at its desired level of use.
Optional ingredients can be added at any convenient
point. If desired, the addition of the various ingredients
can be done simultaneously, sequentially, or incrementally.
However, the general procedure as just outlined is preferred,
i.e. addition of the halogenated phenol to the alcohol or
other coupling agent, followed by addition of the resin pre-
polymer and subsequent dilution with water. Borax and other
buffers can be added last.
In treating woodt the compositions of the type just
described will be prepared at an appropriate solids content
3'~8
level (i.e. the appropriate level of dilution) and used according
-to the following general steps:
1. Wood to be treated is selected for treatment.
The wood will be debarked and usually precut (e.g. dimenslon
lumber) or preshaped (e.g. poles or fence posts). It
may be green or air-dried or force-dried and will often
be in the form of poles or dimension lumber. The ability
to -treat green wood (i.e. wood containing a significant
amount of natural moisture) is an important advantage
of the present process.
2. The wood to be treated is placed in an evacuation
zone and exposed to pressures which are below atmospheric
pressure (e.g. at a vacuum of at least 20 inches of
mercury, preferably at least 26 inches of mercury) Eor
a time sufficient to remove moisture and other volatiles
from the wood. Vacuums of at least 28 inches of mercury
are preferred. This reduced pressure is usually achieved
slowly over a period of from 10 to 120 minutes (e.g.
15 to 60 minutes).
3 The wood is then immersed in or flooded with
the treating composition while still under reduced pressure.
During this phase of the treating, additional water
and volatiles escape from the wood and the treating
solution begins to penetrate into the wood.
4. The pressure within the treating chamber or
zone is then gradually increased to a level above atmos-
pheric pressure (e.g. often to a level of 25 psig-500
-- 10 --
psig, usually 50 to 350 psig) to thereby increase the
penetration of the wood by the treating agent. Typically,
this step is continued until the treating agent has
substantially completely penetrated into -the heartwood,
although -the process could of course be stopped short
of that point if a lesser treatment is desired.
5. To further improve and enhance -the depth and
quality of penetration of the treating solution into
the wood, the vacuum and pressure cycle can be repeated
as many times as desired (e.g. 2 -to 25 or more cycles).
When the process is cycled in this fashion, it is helpful
if the maximum pressure during each cycle is increased
each tirne by, for example, lO-lO0 psig above the maximum
pressure of the previous cycle (usually a pressure increase
of 25% to lO0~ over the previous pressure). This appears
to prevent the rupture and collapse of the wood cells
near the surface of the wood.
6. Recovering treated wood from the treating
chamber.
Note that the vacuum and pressure phases of the treating
cycle should be achieved gradually to avoid destruction of
the cell walls oE the wood. Thus, while speed is important
for commercial purposes (e.g. output and economics), too
rapid a change in pressure can be harmful. Consequently,
the optimum cycle rates, pressure and vacuum should be determined
for each set of operating conditions to be used (e.g. size,
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shape and type of wood to be treated; composi-tion of the
treating compositions; temperaturei etc.).
Since it is the intention of the present process to
cause or permit the resin prepolymer to cure after it has
penetrated deep within the wood, the conditions of treating
including the choice of resin prepolymer should be made so
as to permit easy penetration of the wood followed by a rather
rapid curing of the prepolymer after penetration. Although
a variety of techniques including the use of latent catalysts
might be used to accelerate the curing of the prepolymer,
it has been discovered that one very convenient method of
accelerating the curing of the prepolymer after the treating
solution has deeply penetrated into the wood is to heat the
treated wood. Although this heating can be accomplished
in a variety of ways, the direct application of steam to
the treated wood is preferred. This method requires no special
chemicals and avoids the problems of premature curing that
are sometimes associated with the use of catalysts. Moreover,
the use of steam is convenient, economical, environmentally
acceptable, and does not cause any clean-up problems.
The present process is likewise non-polluting and does
not produce hazardous by-products. When the prepolymer has
cured in place, it serves to lock in the halogenated phenol
and other optional ingredients, retard or prevent leaching
or bleeding of the halogenated phenol and other optional
ingredients (e.g. the organo-metallic compounds), and strengthens
the wood. It is theorized that the prepolymer reacts with
hydroxyl groups present in the cellulosic chains of the wood.
If so, this may account for ----------~-----~------------
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why the treated wood becomes so resistan-t to microbiological
deterioration and insect attack.
The present invention will be further understood by
reference to the following Examples that include a preferred
embodiment. Vnless otherwise indicated, all parts and percentages
are by weight.
Example 1
Ten parts of pentachlorophenol (commercial grade including
the normal impurities) was mixed with 35 parts of methyl
alcohol. Next, 20 parts by weight of an aqueous phenol formal-
dehyde prepolymer (Hooker resin 29765 having a molecular
weight [weight average] of 141.9; a product of Hooker Chemical
Company) is slowly added to the mixture. After the addition
is completed, the liquid concentrate is further diluted with
35 parts of water.
Rxample 2
The procedure of Example 1 is repeated. However, one
part of a nonionic surfactant (Triton* N-57, product of Rohm
& Haas Company) is added to the mixture of methyl alcohol
and pentachlorophenol prior to diluting that mixture with
water.
Example 3
The procedure of Example 2 is repeated wi-th -the further
addition of one-half part of a 10 mole solution of borax to
the completed mixture.
*Trade Mark
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Examples 4-7
In Table 1 which follows are shown the approxima-te formulas
for several treating compositions which may be made according
to the present invention following the general procedure
of Example 1.
Ta_le 1
Example Example Example Example
Ingredlent 4 _ 5 6 _ 7
10 (a) resin prepolymer 7.5 4.3 10 5
soli~sl/
(b) pentachlorophenol 5.5 6.6 3 2
(commercial grade)
(c) methanol 19.025.6 15 20
(d) water 68.061.8 71.767.5
20 (e) surfactant2/ -- 1.0 0.251.5
(f) borax3/ -- 0.7 0.025 --
(g) copper-8-quinoli- -- -- -- 4
nolate solids
___________________.__
1/ from an aqueous phenol formaldehyde prepolymer having
a solids content of 70% and a molecular weight (weight
average) of 141.9; a product of Hooker Chemical Co.
2~ nonylphenoxypolyethoxy ethanol; Triton N-57, a product
of Rohm & Haas.
3/ from a 25% solids solution; a product of Mooney Chemical
Co.
Example 8
Wood (Douglas fir) is treated with the treating composition
of Example 4 by -the following method.
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The wood is a green Douglas Fir pole 10 inches in diameter.
This wood is placed in a pressure treating vessel and the
air is evacuated over a period of 20 minutes to a vacuum
of 28 inches Hg. This reduced pressure is maintained for
30 minutes to permit water and volatiles in the wood to be
removed. Next, the treating solution (Example 4 above) is
pumped into the treating chamber so as to -totally immerse
or submerge the wood in the treating solution. The reduced
pressure is maintained for an additional period of 90 minutes.
Thereafter, the pressure is allowed to increase to 25
psig and held for 40 minutes. The pressure is slowly released
and the vacuum reapplied for 10 minutes. The pressure is
then raised to 50 psig for 40 minutes. Then, the vacuum
is applied for 10 minutes followed by pressure at 75 psig
lS for 40 minu-tes. Then another cycle of vacuum for 10 minutes,
followed by a pressure of 100 psig for 40 minutes. Then
the vacuum is reapplied for 10 minutes followed by pressure at
150 psig for 8 hours. Then, the vacuum is reapplied Eor
10 minutes, followed by 240 psig pressure for 60 minutes.
Then, a final vacuum cycle for 20 minutes. Then the pre.ssure
is returned to atmospheric pressure and the pressure treating
vessel is drained.
~ fter the treating solution is drained from the pressure
treating vessel, but before removing the poles, steam at
15-50 psig is optionally added to the vessel for 15-30 minutes
(depending on size). The purpose of the steam is to accelerate
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the curing of the resin prepolymer which has penetrated into
the wood~
Then, the -treated wood is removed from the pressure
treating vessel.
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